SAN DIEGO — It is not just that the Yankees lost David Robertson. As with Robinson Cano last year, the Yankees for a second straight offseason lost a rare homegrown star to another American League team.

This is a trend. Not just for the Yankees, but for the sport.

Jon Lester could do much to break the pattern sometime Tuesday by going to a National League team such as the Cubs. But the early results show the AL teams are doing much of the heavy lifting.

Robertson became the 16th free agent this offseason to reach agreement for at least $10 million in total. Just four of those were by NL teams, with just Nick Markakis (Orioles to Braves) and Jason Hammel (A’s to Cubs) migrating from the AL to the NL. Pablo Sandoval, Hanley Ramirez, Russell Martin, Adam LaRoche and Zach Duke all exited the NL for the AL.

This exemplifies how many AL teams think they can contend for the playoffs — or more — next season. It used to be nearly as many clubs came to the Winter Meetings as hopeless sellers as motivated buyers. But the sellers are far fewer than the historical norm. Which, in a way, explains what the A’s are doing.

At its core, the Moneyball ethos is not about on-base percentage. It is about capitalizing on inefficiencies in the market. A’s general manager Billy Beane has always said that if Oakland does what every other team is doing, it almost definitely will fail because the A’s will do it with far less money.

A’s GM Billy Beane unloaded righty Jeff Samardzija (above).AP

Thus, when the business in general zigs, Beane’s A’s have zagged. If AL clubs are in all-out build-up mode, Oakland will not only break down, but try to take advantage of the build-up.

Notice the A’s have made three big trades — all to AL teams making pushes to get into the playoffs next year: Josh Donaldson to the Blue Jays, Brandon Moss to the Indians and Jeff Samardzija to the White Sox.

When Beane surveyed the landscape, he saw the Angels finished 11 games ahead of the A’s and were in position to be strong again in 2015. The Mariners followed Cano last offseason with the signing of Nelson Cruz this offseason — and they are not done. The Rangers were artificially poor last year based on an historic injury strike. Even the Astros have a burgeoning pool of young players emerging and now seem willing to augment using cash.

Aside from the Twins and the Astros, there is not an AL team you feel strongly will finish under .500 next year. Even Oakland has retained strong pitching. Plus, Beane is competitive and never fully concedes a season. It would surprise no one if he goes into the free-agent market with the money saved to date and is aggressive in the one- and two-year signing areas, getting players who either help him compete or are easily tradeable come July.

At this moment, all five AL East teams probably feel they can win the division. It was not long ago that those around the Yankees and Red Sox simply conceded to those two giants. But now the others in the division have seen their vulnerability. The Orioles did a late buildup last offseason and won the AL East. The Blue Jays see a window now and are going for it with Martin and Donaldson.

The Yankees are seeing just how difficult it is going to be to make the playoffs even with two wild cards going in each league. They have strong competition from four teams in the East, and it spills over as the other divisions chase the wild card, too. Last year was just the first time since 2006 (and just the second time since 2002) that two AL East teams did not make the playoffs.

The Mariners didn’t make the playoffs. But with Cano, they did finish with a better record than the Yankees. Will we say the same about the White Sox and Robertson this year?